Author Topic: BSA A7 Bobber Engine rebuild  (Read 180 times)

Offline Degsy

  • A's Good Friend
  • ***
  • Join Date: Feb 2021
  • Posts: 43
  • Karma: 1
BSA A7 Bobber Engine rebuild
« on: 13.04. 2021 15:34 »
In January this year I bought a neglected BSA A7 based Bobber that had been sitting in a back yard for a few years.  The owner had moved it to his front garden in the hope that someone would see it and buy it, I was that person. 
I had recently finished my Norton Commando restoration  from a box of bits and was not planning on another project, but I frequently walked past this bike and could not stand to see it rusting away, so I knocked on the door and asked if it was for sale.  It was built in 1980 from a 1952 engine and a 1953 frame, so no viable original bike was harmed in the making of this Bobber.  I am finding lots of problems, but that is OK as it makes things interesting and I paid a fair price considering the condition.

When I got it home turned out the engine was seized, I thought it was kicking over but the clutch was so full of oil and slipping (another story), I should know better and take the plugs out to have a look.  It needed a new cylinder barrel as the internal rust was too deep for the one remaining rebore it had left.  I was amazed to find a replacement barrel locally from a little old lady who lived in a little cottage down a country lane with a barn full of old motorbike cylinder barrels (a story for another time).

Things are progressing well and I did not plan to strip down the bottom end but then decided to clean out the sludge trap after reading the advice on the forum and I am glad I did because the big end bearings don’t look good (see photo). 
I had problems getting the sludge plugs out (see recent thread), the plugs were too far in and one of them was so far in it was blocking the oilway in the crank web,  but the oilway had been enlarged to avoid this, why the plug was not relocated correctly flush instead I don’t know.  With the crank web oilway enlarged I do not think the recessed plug could cause significant oil starvation and explain the worn big end bearings, however I have now found something else. 

At the bottom of the crank case next to the oil drain plug I found a large strong disc magnet (see photo) seemed like a good idea when I found it.  The magnet was underneath a steel mesh grill and held to the steel bottom plate (scavenge cover) through its own magnetism.  I have read in this forum advice regarding an oil flow problem, someone said that magnetic sump plugs could possibly hold the ball bearing valve in the scavenge pipe shut.  The magnet I found is much stronger than a magnetic sump plug and is located a lot closer to the scavenge pipe with its little ball bearing vale.  I wonder if this was causing oil flow problems? What do you think?

See Photo: There is a mesh screen crudely welded to a plate, it does not look standard and there are two beads of weld on the bottom plate whose only function I can think of is to hold the magnet off the plate so there is a gap underneath perhaps allowing a larger surface area of the magnet in contact with the oil, but this will also hold it closer to the scavenge pipe ball valve.

Degsy

Online RDfella

  • Wise & Enlightened
  • *
  • Join Date: Aug 2017
  • Posts: 1329
  • Karma: 10
Re: BSA A7 Bobber Engine rebuild
« Reply #1 on: 13.04. 2021 17:23 »
Degsey - your timing-side conrod may be bent, given the wear indication of those shells.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline Degsy

  • A's Good Friend
  • ***
  • Join Date: Feb 2021
  • Posts: 43
  • Karma: 1
Re: BSA A7 Bobber Engine rebuild
« Reply #2 on: 14.04. 2021 15:07 »
Thanks RDfella
I'll look into getting the rods checked, but I find it hard to believe the shell bearings will always wear evenly if there is some oil starvation.
I have measured the crank journals and they appear to show no uneven wear and are within wear limits for the -020 second regrind so I hope to stick new -020 shells in.

Degsy